A typical radio frequency identification (RFID) tag has a memory configured to store data, such as data that uniquely identifies the tag among a plurality of RFID tags. An RFID reader can be used to communicate with RFID tags over a radio link. Thus, the identity of the tag and the object labeled by the tag can be determined by the RFID reader in an automated process.
In a typical RFID system, an RFID reader is configured to interrogate the tags via radio frequency electromagnetic waves. The RFID reader broadcasts commands using a beam of electromagnetic wave. In response to the interrogation signals from the RFID reader, an RFID tag can be selected to produce a radio frequency response signal.
An RFID tag may be passive, semi-passive or active, depending on the power source of the RFID tag and how the response from the RFID is generated.
A passive RFID tag does not have an internal battery or power source. A passive RFID tag operates using the power drawn from the interrogating electromagnetic wave. A passive RFID tag provides responses through modulating the interrogating electromagnetic wave backscattered by the tag.
A semi-active RFID tag operates using the power drawn from its internal battery or power source. A semi-active RFID tag provides responses through modulating the interrogating electromagnetic wave backscattered by the tag.
An active tag that has an internal battery or power source, using which a separate transmission signal is generated to provide the response. The response signal is generated independent from the interrogating electromagnetic wave.
Radio frequency identification (RFID) tags are used in a variety of applications, such as tagging vehicles on toll roads, tagging shipping containers, quality control on assembly line conveyor belts, and monitoring tactical military equipment maneuvers, etc.
RFID systems typically operate at Ultra High Frequency (UHF) band (300-1000 MHz) or lower microwave bands (e.g., 900 MHz to 2.45 GHz).
Radar systems have been used by police, air traffic controllers and the military for monitoring the presence, distance, direction (from the radar antenna) or speed of individual vehicles or objects, such as airplane and automobile. A traditional radar system detects the presence of an object based on echo of radio frequency signals reflected by the object, measures the distance to the object based on the time delay of a radio frequency signal reflected from the object, in some cases measures the direction from the radar antenna and/or determines the speed of the object based on Doppler shift in a radio frequency signal reflected from the object.
A traditional radar system transmits pulses of electromagnetic waves to detect presence and/or distance. A pulse typically includes an electromagnetic wave having a frequency ranging from below 1 Giga Hertz to over 30 Giga Hertz. An example of the duration of a pulse is in the order of 1 microsecond. Pulses of electromagnetic waves are transmitted repeatedly to detect echoes from different directions and/or to obtain repeated measurements. After the radar transmitter sends the pulse, the radar receiver is used to detect the echo of the pulse until the radar transmitter is used to send the next pulse.
A continuous-wave radar system transmits a continuous electromagnetic wave while its receiver detects the reflected wave. Through detecting the Doppler shift in the reflected wave, the speed of the object that reflects the wave is determined.
Doppler effect is the change in the observed frequency of a wave, as perceived by an observer, when the source of the wave is moving relative to the observer. The frequency observed by the observer increases if the source of the wave moves towards the observer; and the observed frequency decreases if the source of the wave moves away from the observer.
In some continuous-wave radar systems, the electromagnetic wave is modulated in frequency or phase such that the frequency or phase modulation pattern in the received wave can be compared to determine the time delay in the received wave. Thus, such a continuous-wave radar system can also determine both the range and the speed of the object along the line between the object and the radar antenna.
Various radar systems are used in various frequency bands. For example, coastal radar systems operate in the frequency band of 3-30 MHz, long range air traffic control and surveillance radar systems in the frequency band of 1-2 GHz, terminal air traffic control radar systems in the frequency band of 2-4 GHz, airport radar systems in the range of 8-12 GHz, police radar guns in the range of 8-40 GHz, etc.